There are numerous patents related to this type of device. The distinguishing features lie in the method or methods used to sense the movement of the device to the writing or marking surface. All such devices employ a button, contactor, or force sensor to sense when the apparatus is in contact with the writing surface. The combination of the above data can be used to record the movement and marking of the pen.
One method attempts to measure forces applied to the pen tip or pen cartridge by using strain gauges, force sensors, piezo-electric devices, or the like (U.S. Pat. Nos. 4,751,741, 5,215,397). These forces are used to calculate the movement of the device across the page.
Another method uses accelerometers or gyroscopes to sense acceleration and deceleration of the pen (U.S. Pat. Nos. 188,392, 5,247,137, 5,517,579).
An optical technique is used to image a known pattern on the page and use this information to calculate movement of the apparatus in (U.S. Pat. No. 5,226,091).
Finally there are also approaches that use a combination of the above sensing techniques. These include a combination of force and optical sensing (U.S. Pat. No. 5,215,397) and a combination of acceleration and ultrasonics (U.S. Pat. No. 5,517,579).